The subject invention relates to building flooring systems and in particular to a polymer-based composite structural underlayment board and an improved building flooring system that includes a plurality of the polymer-based composite structural underlayment boards overlaying and secured to a building subfloor to form an improved underlayment layer over the building subfloor and a nonstructural finish-flooring layer overlaying and bonded to the underlayment layer. As used in this specification and claims, a “nonstructural finish-flooring layer” is a flooring layer formed of: hard, rigid tiles (e.g. ceramic tiles); thin, flat or substantially flat manmade or natural hard rigid stone slabs (e.g. marble slabs and flagstones such as sandstone and shale slabs); linoleum; and resilient tiles (e.g. vinyl tiles).
In current building flooring systems, the nonstructural finish-flooring layers of the systems may be bonded directly to the upper surfaces of the building's subfloors, e.g. concrete subfloors. However, typically, current flooring systems include underlayment layers formed of plywood boards, hardboards, particleboards, gypsum boards, or fiber cement boards. The boards of these underlayment layers are secured to the upper surfaces of the building subfloors and the nonstructural finish-flooring layers are bonded directly to the upper surfaces of the underlayment layers. While these underlayment boards perform satisfactorily, the physical properties of these underlayment boards, such as but not limited to their weight, handleablity, cutablity, durability, flame spread rating, water absorption and/or fungus growth characteristics, etc. can present problems during the installation of the underlayment layer and over the anticipated service life of a flooring system. Accordingly, there has remained a need for an improved flooring system which utilizes an underlayment layer that is formed by underlayment boards that can be relatively light in weight, that are easy to handle, and that are easily cut at the job site to form the underlayment layer. There has also remained a need for an improved flooring system that includes an underlayment layer made of underlayment boards that are strong and durable, that absorb and retain very little moisture, that are fungus growth resistant, that are flame spread resistant, that have relatively good thermal and acoustical properties, and that have good bonding surfaces for bonding a nonstructural finish-flooring layer to an underlayment layer formed by the underlayment boards.